Rotary wing aircraft



R. HAFNER ROTARY WING AIRCRAFT Filed oct. 1s., 1937l 4 sheets-sheet 2 Filed oct. 16,l 1937 |NVENT9IQ ATTN.

Dec. 9, 1941.v

ROTARY WING AIRCRAFT R. HAFNER 2,265,366

Filedoct. 1e, 1957 l 4 sheets-sheet s www H er INV TOR @@SWJQ,

Patented Dec. 9, 1?

anni miner, vienna, Amma Application October 16, 1937, Serial No. 169,382 In Greatritain December 11, 1936 14 Claims.

This invention relates to the sustaining blades of rotary wing aircraftl of the kind comprising a plurality of blades vradiating from a common rotary hub. andvhas particular though not exclusive reference to aircraft of this kind in which each blade is adjustable during flight about a pitch-change axis extending in the direction ofv its span. y

It is an important object'of the invention to provide a construction in which the vinevitable fluctuations in the aerodynamical influence on a blade during flight may be arranged to produce a minimum of disturbance in the, flying control m'eans and/or general structure of the aircraft itself. Other important results are also achieved by the invention, and will be referred to in due course. A

It has been proposed, in rotary wing aircraft of the kind referred to, and having each blade journalled to the hub about an induzidual pitchchange axis, so to construct each blade that the will of course be understood that the aerofoil sections chosen for the blade will be such that the positions of their centres of pressure will not be substantially affected by normal variations in blade incidence, for example certain sections having a slightly refiexed trailing edge may be ,Y used.

general centre of lift of the blade as a whole is not displaced by changes in incidence, and to arrangefor the pitch-change axis to pass through this centre of lift and the centre of gravity of the blade as a whole.

The purpose of this arrangement was to avoid lift-mass couples about the pitch-change axis being caused by changes in-pitch and reacting on the controls, but it took no account of the cyclic variation in the lift distribution along the blade. during forward flight, due to the cyclic variation in the relation between the rotary speed of the blade and the speed of the steady ight wind past it. Consideration will show that a span-wise os- A the rotational axis of the hub, at least in thev mean flight ldisposition of the blade respectively cillationof' theeneral centre of lift is causedv n thereby, which the general case will have a component-in the direction of the chord (e. g. the centre of lift will oscillate along .a curve) whether or not the arrangement is such thatthe position of this centre is unaffectedl by changes in 4:ln-

cidence. y

According to the present invention a sustaining blade ls'so constructed that, with changes in the lift distribution along its span,`its general centre of lift moves along a lsubstantially straight spanwise line, or axis of balance as it will hereinafter Yelements.

Preferably, for reasons which need not be considered here, the axis of balancepasses not only through the general centre of mass of the blade as a whole but also through substantially all the individual centres of mass of the said sectional New it will be appreciated that the axis of balance in the blade of the invention always passes through the mass and lift centres of the blade as a Whole, whatever their relative posiy tions at any instant. The invention therefore 'provides a predetermined vaxis of reference in 4 the blade about which there are no fluctuating moments during flight, and the blade root attachments and/or flight control means may be so arranged in relation to this axis as to eiect a general improvement inthe smoothness of operation of the aircraft.

- For example, in aircraft in which each blade is angularly adjustable during flight about an individual pitch-change axis, the latter may be arranged to coincide with the axis of balance of the blade, thus ensuring that no vibratory reactions will be transmitted to the pitch control means.

Additionally or alternatively, matters may be so arranged that this axis of balance intersects to the hub. (This may be ensured. where the blade root articulation includes a drag axis allowing the blade to lag and advance somewhat during rotation, by arranging for the axis of balance to intersect such drag axis.) As a result, since there are no'iluctuating moments about the axis of balance, there will be no tendency for be termed; which passes through the general v centre of mass of the blade.

The successive sectional elements of the aero structed and mutually disposed that their means centres of pressure lie on an axis of balance; lt

. foil or lifting part of the blade (that is tosayf any individual blade to impose torsional vibrations on the hub and thence on the aircraft structure, or on the flight control means where this operates by tilting the hub and is sensitive or reversible It will be appreciated that the foregoing takes no account of the lift-mass couple acting on the blade in a plane including the axis of balance of the' blades and transverse to the chord thereof. This couple maintains an upwardiiexure 'of the blade towards its tip, vand its uctuations are balanced by variations in its vertical reaction on the hub. With the object 'of minimizing undesirable effects due to this. according toa further feature of the invention, the blades may be hinged about napping axes all intersecting the rotational axis of the h ub, preferably at a single point. In such a case. the varying vertical reactions referred to will all act along the said rotational axis and their effects will be purely additive: they will not combine to produce vibratory couples on the hub or aircraft structure, and their resultant will only react on `a hub-tilting night control if the tilting axis or point is appreciably offset from the rotational axis of the hub. Moreover. in the preferred form of aircraft laccording to the invention three. or a multiple of three, blades are evenly spaced around the hub; insuchcases the fluctuating reactions referred to tend to balance out'in the manner of'three simple harmonic functions 120 out of-phase.

A very satisfactory practical form or sustaining rotor incorporating the foregoing features may be arrived at by associating three yblades conv structed in accordance-with the invention with escasos L' the hub and flight control means set out in our o British Patent No. 435.818, the arrangement being 'such that the axis of balance of -each blade coincides with'the torsionally` resilient tie member which lies within the blade root. anchors the blade radially to the hub, and is torsionaily flexed .by pitch changes.

An example of such an arrangement is illus` trated by Figures l to 3' ofthe accompanying drawings. of which o Figure l is a plan view showing thehub and parts of one blade; A

Figure 2 is a side elevation thereof, largely in section:

Figure 3 is a plan view, on a smaller scale, of the entire blade;

Figs. 4 to 6 are diagrams of a blade profile with,

the neutral torsional axis disposed in three different positions, respectively. relative to the axisv of balance;

Fig. 'l is a diagrammatic cross-sectional view of a partly completed blade accordingl to the invention and of a mould which may be used in the initial stage of a process for producing such blade from synthetic resins:

l standing cylindrical inner and outer walls 2| and 22. The inner wall 2I is journalled upon a tubular axle 23 through which passes a vertical control column 24. Y

Three blades are provided. which are pivoted to the hub. about individual napping axes P-P (Figure l) which arejcoplanar and intersect the rotational axis of the hub at a common point F. Flapping Pins 2l are arranged on these axes and are carried at their inner and outerends by the walls 2| and 22 of the hub. and the blade roots carry forks. which nest in regularly interlapping relationship inthe annular space between these walls and are pivoted at their ends to the respective pins 2i., The fork arms of each blade are cut away or apertured at 21 (Figure 2) to give passage to the flapping pins relating to the other blades.

The outer wall 22 of the hub is cut away at 2l in'three places to accommodate the shanks of the forks 24 and these shanks carry vertical drag the supporting arm $0. At its outer vend the tie pivot yplus 2l about each of which is pivoted the knuckle end of a short tubular blade-supporting arm Ill. An adjustable friction damper 2i is associated with this pivot.

The root .portion of the blade spar consists oi tial friction between thjese parts when the air ma is in sight.

The outer end of the tubular spar root is se- .A cured to the inner end ofthe blade spar proper' (84) which is much thicker-walled than the root portion 32, and may in factbe solid. The spar has thus at its root a greater depth but less weight per unit length than intermediately in its length towards the tip; this arrangement gives not only the requisite root stiffness for support of the blade when at rest, but also the best distribution of mass for the storage of a large amount of excess kinetic energy at a reasonable excess rotor speed. The second-mentioned feature is of importance in effecting a direct take ou by altering the general pitch-of the rotor so as to convert such excess energy into positive lift, and it may be noted that the relative thinvness of the spar proper contributes in this respect, in enabling the fast-moving blade proper to be constructed with va, thin aerofoil section having a low drag.

The -blade'is anchored internally and radially to its root articulations by a torsionally resilient tie member 35, in the form of a steel rod swaged up and externally threaded at its ends. and lying along the common axis of the spar root 32 and rod 35 is screwed into a tapped axial boring in the blade spar 34 and secured against vrotation by a nut and tapered collet arrangement indicated at ll, and at its inner end it engages with an internal screwthread in the supporting arm 30 and is similarly secured by a screwed plug and taper pin assembly 31.

The blade pitch is controlled by a radius arm ll which is pivoted about a'vertical axis to a clamp I9 secured around the spar root and projects generally forwardly of the blade in the di of the spider arms is rigidly-secured to the rotary head 40 in the manner'ofa master arm, but the other two arms are pivoted at-their roots about vertical axes to the rotary head; the mutual angular disposition of these arms can thus vary slightly in accordance with movements o the blades about their drag pivots.

'By sliding the control member 24 vertically over a small range the pitch of the rotor may be adjusted as a whole, whilst ifthe said column is 'tilted in any direction about the ball joint 43 the geometrical blade pitches are varied cyclically or dierentially' and settles down to rotation in the same general tilted plane as theball joints 42. It should be noted that the joint 43 is so placed that tilting 0f the control assembly.v

takes place substantially about the point F, which is in turn the point about which the true rotary axis of the blade system is tilted.

The latter curve somewhat aaeasee l Variation of the pitch of a blade is opposed only by the torsional resilience of its tie rod, there being no frictional restraint as would be the case if thrust bearings were used at the blade roots. This is of particular importance ai: times when the blade pitches are changing "differentially,"

since each blade anchorage produces a harmonically varying reaction on the control assembly instead of a frictional reaction which would be constant in value and subject only to rapid reversal. Moreover, where there are three or a multiple of three blades these harmonic variations will sum to zero at any instant, leaving only (as consideration will show) a basic steady reaction urging the control system resiliently to its central position.

As shown by Figure 3, each blade is tapering or lancet-shaped inl plan, and of course its spar and thickness taper correspondingly. Such a shape is that most suited to the special reon the axis of balance in a blade according to the invention.

Now in this (or any) section of a blade there is one point, positioned in the chord according to the blade structure.' where an applied vertical thrust will cause equal degrees of flexure of the leading and trailing edges, i. e. will cause no change in incidence. This point is marked O in Figures 4 to 6, and its locus along the blade is what has been above referred to as the neutrai torsional axis" of the blade.

In Figure 4 the point O is behind the -axis of balance or lift centre C, and an increase in lift L, besides upwardly flexing the blade as a whole, will therefore twist it in such a sense'as to increase its angle of attack and so aggravate the original disturbing factor. At the same time the mass inertia of the blade. acting downwardly at a point on the axis C further towards the quirements of rotary sustaining blades (forl reasons which need not here be discussed), and, in accordance with what has previously been set forth'herein, is so built up on its spar 'that the centres yoi. mass and means centres of pressure of all its sections lie on the axis B-B of its spar. This axis is therefore the axis of balance of the blade and evidently intersects the drag and flapping axes of the blade and also (in the mean flight position of the blade) the general rotary axis at the point F.

In the abovev arrangement, thereof, the general rotary axis of the rotor and the flapping axes, axes of balance, and pitch-change axes of the blades all mutually -intersect at a single focal point F, about which a central flight control member is universally tiltable to vary the blade pitch angles cyclically or diiferentially,

and the general aerodynamic Ymerit of such. an.

arrangement will be evident.

The various expedients above set outl have the eiect, inter alia, of minimising or obviating torsional reactions at the blade root and, conversely, any torsional oscillation or flutter ofv the blade which would tend to set up such reactions.

There is, however, another type offiutter to which blades are liable during flight, in which the blade is oppositely twisted, at any instant,

"at different points in its span, no reaction being in this case set up at the root, and it is a further important object of the invention to provide a blade which is not liable to internal flutter of this kind. This is particularly desirable since the sustaining blades of rotary Wing aircraft are unlike fixed wings and airscrew blades in having no great torsional stiffness, yielding resiliently to forces tending to twist them. Any consequent torsional flutter is therefore: likely to be of appreciable amplitude and to have a serious effect on the efciency of a blade.

" Y`v'with the object of avoiding such intemw flutter, a blade according to the invention (e. g. that of Figure 3) may be further soconstructed that its neutral torsional axis (hereinafter defined) is at or slightly in front of the axis of balance lthereof. The eiect of this may be explained as follows:

In each of the diagrams formingFigures 4 to 6 of the accompanying drawings, the dotted outline represents the blade section, at the lift centre of the blade as a whole, ina steady flight position, and the full outline this section as deflected by a fortuitous increase in the lift L acting at the lift centre C, which latter is of course root, will twist that part of the blade oppositely tov decrease its angle of attack and aggravate the downward component acting upon it.

Thus the inevitable fluctuations of lift on this blade cause twisting couples to be applied thereto about the neutral torsional axis, which'build up against the torsional resilience of the blade and set up internal flutter thereof. It may be remarked that Figure 4 is typical of the customary rotary blade, in which the wood or metal surface (necessary to withstand the high aerodynamic loading) forms a stress-carrying skin and causes the axis O to lie towards the midspan position.

Figure 5 illustrates the behaviour of a blade according to the inventionA in which the axes O and C are approximately coincident. In this case variations in-the lift and mass forces will obviously causepure flexing of the blade with- -out 'torsional stress therein, and such a blade 0 may be regarded as having neutral" stability from the present point of view. t

In Figure 6 a modified blade according to the invention is shown in which the axis O is slightly in advance of the axis of balance C. In this case the blade will be slightly twisted, but always in such asense as to offset the effect of initiating force; such a'blade therefore has positive stability from the present Viewpoint.

It will of course be understood that Whilst the blades of Figures 5 and 6 are in any case a distinct advance on that of Figure 4, it will always be of advantage to use aerofoil sections in which the centre of pressure is substantially stationary, or moves slightly in the stabilising direction,

' with alterations in incidence, for example certain sections having a reflexed trailing edge.

The above principle may obviously be carried into practice in a. number of alternative ways. In the case of hollow stressed-skin blades the nose portion may be reinforced to the requisite extent by an extra layer or layers of material in order that it may predominate in taking the bending loads on the blade, this of course'bring ing the neutral torsional axis forward as required. Alternatively the nose portion may com- 'prise material having a higher modulus-of elas- A ticityvthan the remainder, for example steel as against as aluminium, magnesium or other light alloy for the rest of the blade. Both these expedients will also bring the mass centres of the sections correspondingly forward; any separation of these from the mean centre of pressure l will therefore be slight, and a very small amount of nonstructural mass suitably ineorpgrated in the blade will suillce to correct it.

structional form of blade according to the inby a twostage moulding process, Figures 'l and 9 `being diagrammatic cross-sectional views of the Where blades according to the invention are 'built up on a load-carrying spar situated at the axis of balance of the blade, the built-up portion may be formed of a material whose modulus of elasticity is so much lower than that of the spar as not to relieve the latter appreciably of its loads. 'I'he neutral torsional axis of the composite blade will thus be that of v the spar, and consequently will remain at the axis of balance as required. Such a blade might be regarded as a structural spar or beam carrying a nonstructural aerofoil fairing for imparting lift thereto. As regards the material for-this fairing, certain mouldable materials ofthe synthetic resin or plastics class have physical properties which appear to render-them entirely suitable for this purpose in associationwith a steel spar, and we therefore propose to produce a preferred convention, for example, that of Figure 3, by moulding such material about aspar of this kind.

Figures '1 to 9 of the accompanying drawings illustrate the formation of a-blade in this way,

mould and its contents in the first and second stages respectively, and Figure 8 being an en larged perspective view of a length of the blade before the second and final stage.

The mould for the first stage comprises upper and lower dies I and 2 and an interposed c lie or core 3, these being shaped and assembledl as shown in Figure "l, so that the blade is initially formed with a solid nose portion in whichthe steel spar 4 is embedded. During moulding, the spar is positively located between the dies I and 2 by spaced pegs 5 upstanding from sockets in the llower die 2 and fitting laterally into holes spaced along the front of the spar. As a further precaution against the spar moving vertically and weakening the thin moulded covering above and below it, it is located between rows of spaced raised points on the dies'l and 2.

' tion by moulding the blades around their spars has a number of important advantages in practrue not only of blades having a uniform section blade.

senseo naal-shape ready for nishing, for example ironing out or filling in the pin-pricks caused by the spar-locating points l of the dies.

In a blade so constructed, as previously explained, the neutral torsional axis is substantially that of the spar, and may therefore readily be placed at or slightly in front of the axis 'of balanceV of the blade. 'I'he massof the solid nose portion of course more or less balances that of the more extensive but cellular remainder'of the In the particular case shown, it is estimated thatthe mass centres of the blade sections will be' brought thereby almost to their centres of pressure, and the remaining discrepancy willl be'removed by the use of a suitably heavy gauge forthe leading edge wire J to form a non-structural balancing mass. v

It may perhaps also be noted that the density of the `nose portion as compared with the trail'- ing portion, has also the effect of equalising any slight stress-carrying actions by these parts, and so completely eliminating any minor eect` they might otherwise have on the position of thev neutral torsional axis of the blade.

The preferred way of carrying out the inventice, among which may be mentioned the followirm 'Ihe blades may be cheaply and rapidly produced in large numbers and will be exactly similar and interchangeable in all respects. 'I'his is over most of their length but also of those shaped more in accordance with the special requirements of rotary wings, for example, those having a progressive change in section and blades tapering or lancet shaped in plan. l

'I'he moulded material takes a high finish, with a consequent reduction'in drag, and lis superior to wood both as regards homogeneity and freedom from warping and splitting tendencies. Ad-

' ditionally, the latter qualities permit the adoptively of the blade in order -to give the latter strength in tension, in which the moulded material is,apt to bedecient, and in the particular construction shown the leading edge wire 8 is of relatively heavy gauge for a reason subsequently explained.

At the conclusion of this stage the blade has the form shown in Figure 8, with the upper and lower skins behind the spar evenly. separated vover their entire length right back to their trailing edges, triangularl ribs III and angle webs II having been moulded on the underside of the upper skin by appropriate slots in the upper side of the core die 3'. f

The dies 2 and 3 having been removed, the

-projecting pegs 5 are cut off flush and drilled out tion of a simple jolntless structure such as that shown, which is very considerablylighter than an equivalent built-up wooden blade.

A further point which may be noted is that:

I oifset the disruptive inuence'of centrifugal force during flight.

There are many materialsvof the synthetic resin or plastics" class suitable for the construcsubstantially eliminate the'distractlng flickering the trailing edge, after which the blade is in its 75,

in the pilots eyes frequently present when rotary wing aircraft are flown or landed facing a strong sun.

What I claimand desire to secure by Letters.

Patent of the United States is: v

l. In or for rotary wing aircraft, a sustaining bladev the successive sectional elementsA of whose lifting portion are each of an aeroioil shape having a substantially stationary center of pressure and are mutually disposed with said centers of pressure all on a straight spanwise axis of balance passing through the general center of mass of said blade.

2. In or for rotary Wing aircraft, a sustaining blade the successive sectional elements of whose lifting portion are each of an aerofoil shape having a substantially stationary center of pressure and are .mutually disposed with their individual centers of pressure and of mass all on a common straight spanwise axis of balance.

3. In or for rotary wing aircraft, a sustaining blade the successive sectional elements of whose lifting portion are each of an aerofoil shape havblade the successive sectional elements of whose lifting portion are each of an aerofoil shape having a substantially stationary center of pressure and are mutually disposed with their individual centers of pressure and of mass all on a common straight spanwise axis of balance substantially coincident with the neutral torsional axis of said blade.

5. In or for rotary wing aircraft, a sustaining blade constructed in accordance with claim 4, and being of a tapering or lancet shape in plan view.

6. In or for rotary wing aircraft, a sustaining blade constructed' in accordance with claim 4, said blade comprising a stressed metal skin augmented as to mass and stiffness in front of said axis of balance.

7. In or for rotary wing aircraft, a sustaining blade constructed in accordance with claim 4, said blade comprising a stressed metal skin augmented as to thickness in front of said axis ofbalance.

8. In or for rotary wing aircraft, a sustaining blade constructed in accordance with claim 4, said blade comprising a stressed metal skin and a reinforcement in the nose portion of a metal having a higher specific gravity and modulus of elasticity than the metal of said skin.

9. In or for rotary-wing aircraft, a sustaining blade constructed in accordance with claim 4,`said blade comprising a metal spar at said axis of balance and a blade shape formed around said spar from a material having a modulus of elasticity greatly inferior to that of said spar.

10. In orfor rotary wing aircraft, a sustain ing blade constructed in accordance with claim 4. said blade comprising a steel spar at said axis of balance and a blade shape molded around said spar from a synthetic vresin material.

11. In or for rotary Wing aircraft, a sustaining blade constructed in accordance with claim 4, said blade comprising a steel spar at said axis of balance and a blade shape molded around said spar from a transparent or translucent synthetic resin material.

12. A rotary wing aircraft comprising a plurality of sustaining blades each constructed in accordance with claim 4 radiating from a rotary hub structure and each articulated to said hub structure about a pitch-change axis coincident with its said axis of balance.

13. A rotary wing aircraft comprising a plurality of sustaining blades each constructed in accordance with claim 4 radiating from a rotary hub structure and each articulated to said hub structure about a pitch-change axis coincident with its said axis of balance, and torsionally resilient means anchoring each blade radially to said hub structure against centrifugal force in flight and torsionally flexed by changes in pitch.

14. A rotary Wing aircraft comprising a plurality of sustaining blades each constructed in accordance with claim 4 radiating from a rotary hub structure and each articulated to said hub structure about a pitch-change axis coincident with its said axis of balance and about a flapping axis, all said axes intersecting the rotary axis of said hub structure substantially at a single point.

RAOUL' HAFNER. 

